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in plants adapted to Mediterranean conditions. Such an adaptation may be
most pronounced in summer-flowering species, which are visited by a large
number of nectar-consuming insect species, as is the case with
Thymus capi-
tatus
(Petanidou, 1991).
3.2
Humidity
A considerable part of the Mediterranean region is made up of coastal areas,
where the sea has a dominant effect on terrestrial habitats. Pérez-Bañón
(2000) found that in such habitats it is not primarily the temperature, but the
differential humidity that positively affects nectar secretion, both in volume
and sugar content. Working on
Medicago citrina
, a leguminous shrub in the
archipelago of Columbretes, in Spain, the author discovered that the low
relative humidity had a dramatic effect on nectar secretion. Nectar secretion
was measured (i.e., volume, concentration, and sugar content per flower) on
several mid-March mornings and ambient temperature and relative humidity
were also recorded throughout the day. Amongst all parameters tested, the
ones found to affect nectar secretion were (i) the mean of the maximal tem-
peratures recorded over the 24 h preceding sampling, and (ii) the mean
relative humidity recorded 2 h before sunrise (6:00-8:00). Further analysis
of the data showed that mean relative humidity had a positive effect on both
nectar volume and sugar content, which was more significant than that of
temperature at all flower ages tested. The conclusion is that, in island com-
munities, atmospheric humidity may play a very crucial role in nectar
secretion that is otherwise limited by extreme water drought, evidently more
important than temperature itself (Búrquez & Corbet, 1998).
3.3
Light intensity
Mediterranean plants are generally adapted to high light intensities and their
nectar secretion is not expected to be limited by solar irradiance under nor-
mal Mediterranean conditions. Under unfavourable light conditions nectar
secretion may decrease dramatically. Experimenting on
Thymus capitatus
, I
found that, with a few exceptions, flowers in the sun secreted more nectar of
a higher concentration than flowers growing mostly in the shade, implying
limitation by solar irradiance (Petanidou, unpublished data). When
T. capi-
tatus
plants were grown under typical temperate conditions (i.e., under low
temperature and light intensity), solar irradiance was the most significant
limiting factor for nectar secretion, not low temperature (Petanidou & Smets,
1996). The experiment was repeated with
Ballota acetabulosa
, another labi-
ate species sympatric and co-flowering with
T. capitatus
, which differs in its
microhabitat preference by usually growing in more shaded areas.
Ballota
